Dispersion compensating optical fiber, and communication system comprising same

ABSTRACT

Known dispersion-compensating (DC) optical fibers typically are sensitive to small changes in fiber parameter (e.g., fiber diameter and/or core refractive index), and thus are difficult to manufacture. The disclosed DC fibers are relatively insensitive to small departures from the nominal fiber parameters, and are therefore more manufacturable. Exemplarily, the nominal refractive index profile of a DC fiber is selected such that the fiber supports LP 01  and LP 02  (and typically one or more further higher order modes), and the dispersion is substantially all in LP 02 . The total dispersion is more negative than -200 ps/nm.km over a relatively wide wavelength range. The nominal refractive index profile typically comprises a refractive index &#34;ring&#34; that is spaced from the fiber core.

FIELD OF THE INVENTION

This invention pertains to dispersion compensating optical fiber, and tooptical fiber communication systems that comprise such fiber.

BACKGROUND

Dispersion compensating (DC) optical fibers are known. See, forinstance, U.S. Pat. Nos. 5,185,827, 5,261,016 and 5,448,674. The priorart discloses dispersion compensation by a technique that comprisesconversion of the LP₀₁ (fundamental mode) radiation of conventionalsingle mode fiber to a higher order mode (e.g., LP₀₂), coupling thehigher order mode radiation into a length of DC fiber and, aftertransmission through the DC fiber, reconverting the radiation to theLP₀₁ mode. The DC fiber is selected such that it guides the higher ordermode, and such that the dispersion of the higher order is of theopposite sign from the dispersion of the LP₀₁ mode in the conventionalsingle mode fiber. The prior art also discloses dispersion compensationby a technique that does not require conversion of LP₀₁ radiation tohigher mode radiation, and that instead uses DC fiber that supportspropagation of the signal radiation in both the fundamental mode and ahigher order mode (typically LP₀₂). The latter prior art DC fiber doesnot support propagation in the LP₁₁ mode. See U.S. Pat. No. 5,448,674.

Prior art techniques of dispersion compensation can in principle providean essentially completely dispersion-free (single channel) optical fibertransmission system. However, in practice it is difficult to obtainessentially complete compensation, since known designs of DC fiber arevery sensitive to manufacturing variations, e.g., variations in fiberdiameter and/or refractive index profile. It is typically possible tomaintain the fiber diameter (and therefore all radial fiber dimensions)to within ±1% of the nominal value, and to maintain the core refractiveindex within limits such that Δ is within ±2% of the nominal value. Theparameter Δ is defined below.

Those skilled in the art are well aware of the practical impossibilityof producing optical fiber that is completely free of unintendedvariations of fiber characteristics such as fiber diameter or corerefractive index. Thus, it would be highly desirable to have availableDC fiber that is less subject to unintended variations of fibercharacteristics than typical prior art DC fiber. This applicationdiscloses such a fiber, and communication systems that comprise thefiber.

SUMMARY OF THE INVENTION

In a broad aspect the invention is embodied in a DC fiber (and in anoptical fiber communication system that comprises the DC fiber;collectively "article") that is relatively immune to unintendedvariations in fiber diameter and/or refractive index, and thus is morereadily manufacturable than typical prior art DC fibers.

More specifically, the invention is embodied in an article thatcomprises a length of DC optical fiber that supports radiation ofwavelength λ_(o) (e.g., 1.55 μm) in a fundamental mode LP₀₁ and a higherorder mode. The DC fiber has a nominal refractive index profilecomprising a core of diameter d_(c),nom and maximum refractive indexn_(c),nom, and that further comprises a cladding region thatcontactingly surrounds the core and has a refractive index n₁,nom, thatis less than n_(c),nom. The quantity (n_(c),nom -n₁,nom)/n_(c),nom isdesignated Δ_(nom).

At least a portion of the length of DC fiber has a refractive indexprofile that differs from the nominal refractive index profile,typically due to unintended variations during fiber manufacture. Therefractive index profile of the portion includes a core of diameterd_(c) and maximum refractive index n_(c), and further includes an innercladding region that contactingly surrounds the core and has refractiveindex n₁, with (n_(c) -n₁)/n_(c) designated Δ. At least one of d_(c),n_(c) and Δ differs from d_(c),nom, n_(c),nom and Δ_(nom), respectively.DC fiber having the nominal refractive index profile has dispersionD_(nom) (λ), and the portion of the length of DC fiber has dispersionD(λ) that differs from D_(nom) (λ).

Significantly, the nominal refractive index profile is selected suchthat the length of DC fiber supports LP₀₁ and at least the higher ordermode LP₀₂ at Δ_(o), and D_(nom) (λ) is more negative than -200 ps/nm.kmover a wavelength range λ_(max) ±50 nm, where λ_(max) is the wavelengthat which |D_(nom) (λ)| is maximum. Furthermore, |D(λ)-D_(nom) (λ)| isless than |0.5 D_(nom) (λ)| at every wavelength λ in the range λ_(max)±50 nm, for specified departures of the refractive index profile fromthe nominal refractive index profile. Specifically, the above inequalityis satisfied for d_(c) that differs from d_(c),nom by 1% or less, or forΔ that differs from Δ_(nom) by 2% or less, or for d_(c) and Δ thatdiffer from d_(c),nom and Δ_(nom) by 1% or less and 2% or less,respectively. Still furthermore, λ_(o) is a wavelength in the rangeλ_(max) ±50 nm. As is conventional, the vertical bars around a quantity(e.g., |0.5 D_(nom) (λ)|) signify the absolute value of the quantity.

The DC fiber typically supports, at the wavelength λ_(o) not only LP₀₁and LP₀₂ but also one or more further higher order modes. The latterhigher order modes typically are close to cut-off and are very sensitiveto bending loss, typically resulting in their removal from the fiber. Anoptical fiber "supports" a given mode if the effective index of the modeis greater than the refractive index of the cladding of the fiber.

The nominal refractive index profile preferably comprises a refractiveindex "ring" of inner diameter d₁,nom >d_(c),nom, and outer diameterd₂,nom, and a refractive index n₂,nom selected such that (n₂,nom-n₂,nom)/n₂,nom >0.1%, preferably greater than 0.2%. Furthermore,d_(c),nom and Δ_(nom) typically are selected such that (n_(c),nom·d_(c),nom ·√Δ_(nom) )/λ_(o) is greater than 0.55, indicative of fiberthat supports several modes.

In embodiments for λ_(o) ˜1.55 μm, wherein the dispersion-compensatingfiber is silica-based fiber with Ge-doped core, the nominal refractiveindex profile is equivalently selected such that d_(c),nom ·√Δ_(nom) isgreater than 0.6 μm.

The core, inner cladding region and index ring of the DC fiber willtypically consist of glass formed in situ by any one of the conventionalmethods, e.g., MCVD. Frequently but not necessarily, the index ring issurrounded by an outer cladding of in situ formed glass, having outerdiameter d₃ and a refractive index <n₂, typically n₁. The remainder ofthe fiber typically is pre-existing glass extending from d₃ to the fibersurface. If attainment of minimum loss is not a significantconsideration, then the index ring could be directly deposited on theinside of the cladding tube, i.e., d₃ =d₂.

The outside diameter of the fiber exemplarily is 125 μm. The fibertypically is silica-based, with the pre-existing glass typically beingundoped silica, and the inner cladding and optional outer claddingexemplarily also being undoped silica, or being down-doped (e.g.,F-doped) silica. Since the outer portions of the fiber claddingtypically do not significantly affect the transmission characteristicsof the fiber, we do not distinguish between nominal and actual diametersand refractive indices for these portions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows the refractive index profile of an exemplaryDC fiber according to the invention;

FIG. 2 schematically shows a portion of an optical fiber communicationsystem, the portion comprising a dispersion compensator;

FIG. 3 schematically shows an optical fiber communication system thatcomprises a dispersion compensator;

FIG. 4 shows data on dispersion vs. wavelength for DC fiber according tothe invention;

FIG. 5 shows data on dispersion vs. wavelength for an exemplary priorart DC fiber;

FIG. 6 shows data on delay vs. wavelength for the combination of 100 kmof conventional transmission fiber and a length of DC fiber according tothe invention;

FIG. 7 shows data as in FIG. 6, except that the DC fiber is theexemplary prior art fiber of FIG. 5; and

FIG. 8 shows data on LP₀₂ mode effective index for the DC fiber of FIG.1 and the exemplary prior art DC fiber of FIG. 5.

DETAILED DESCRIPTION OF SOME PREFERRED EMBODIMENTS

FIG. 1 shows the nominal refractive index profile of an exemplary DCfiber according to the invention. As those skilled in the art know,actual profiles typically depart somewhat from the nominal or idealprofile, but the departures typically can be kept within limits suchthat the fiber exhibits the desired characteristics. For instance,actual fibers typically exhibit a central refractive index depressionthat is typically not present in the corresponding nominal profile, yetthe fibers generally perform according to the design specifications.

In FIG. 1, numerals 11-14 refer, respectively, to the core, the innercladding region that contactingly surrounds the core, the refractiveindex ring, and an outer cladding region that surrounds the index ring.The fiber typically comprises a further cladding region that is notshown, and that typically is spaced far enough from the core to beessentially optically inactive. The material typically is undopedsilica, and extends to the fiber edge from a (non-critical) outerdiameter d₃ of outer cladding 14. In a preferred embodiment, d_(c),nom=8.4 μm, d₁,nom =14.55 μm, d₂,nom =25.5 μm, and d₃ is about 30 μm. Thefiber diameter is 125 μm, as is conventional.

In FIG. 1, the core has a step profile, with Δ_(nom) =1.8%. The stepcore profile is chosen to provide substantial dispersion. This choice,however, is optional, and other core profiles (e.g., parabolic profile)could be used if desired, e.g., to minimize signal attenuation. Therefractive index ring desirably has refractive index n₂,nom selectedsuch that (n₂,nom -n₂,nom)/n₂,nom >0.1%, exemplarily, 0.4%, and claddingregions 12 and 14 have refractive index<n₂,nom, typically but notnecessarily the same in both regions. The refractive indices of regions12 and 14 could be that of undoped silica (n_(o)), or could be less thann_(o) due to F-doping. The latter is preferred, since it makes possiblelower core doping.

A further exemplary DC fiber according to the invention has a parabolicindex core (d_(c),nom =12.6 μm, Δ_(nom) =1.8%), d₁,nom =20 μm, d₂,nom=34.6 μm, d₃ is about 40 μm, with fiber outside diameter of 125 μm. Theindex ring has refractive index n₂,nom selected as above, with (n₂-n₁)/n₂ =0.4%, and the inner and outer cladding regions (correspondingto regions 12 and 14 of FIG. 1) have refractive index n₁,nom.

DC fiber according to the invention is designed to primarily use theLP₀₂ mode for dispersion compensation, since near cut-off the LP₀₂ modecan have high (negative) dispersion, desirably more negative than -200ps/nm.km. Fibers that have dispersion less negative than -200 ps/nm.kmare generally not as useful as dispersion-compensating fiber as are moredispersive fibers. Fiber according to the invention can be produced byconventional fiber manufacturing techniques, e.g., by MCVD.

FIG. 2 schematically depicts a DC fiber according to the invention, withassociated input and output means. The combination 20 comprises apredetermined length (typically a km or more) of DC fiber 21, standardsingle mode input and output fibers ("pigtails") 221 and 222, andgratings 241 and 242 selected to act as mode converters. The gratingswill generally be long period gratings. Input grating 241 is selected toconvert incoming LP₀₁ radiation to LP₀₂ radiation, and output grating242 is selected to convert outgoing LP₀₂ radiation to LP₀₁ radiation.Long period gratings are known to those skilled in the art and do notrequire further elaboration. The gratings will typically be formed inshort lengths (231 and 232) of fiber that supports both LP₀₁ and LP₀₂.Exemplarily this could be DC fiber according to the invention. However,at least in principle, the gratings could be formed directly in thepredetermined length of DC fiber 21, with the spacing between the twogratings being the effective length of the DC fiber. In FIG. 2, the "x"symbols refer to conventional splices. The splices between fibers 231and 21, and between 21 and 232, are optional. Splices 251 and 252 arebetween single mode fiber and fiber that additionally supports one ormore higher order modes, and should substantially only couple LP₀₁ toLP₀₁.

The appropriate length of the DC fiber is determined in conventionalfashion so as to compensate the dispersion of a given length ofconventional single mode transmission fiber.

FIG. 3 schematically depicts an exemplary optical fiber communicationsystem 30 that comprises a combination 20 according to the invention.Transmitter 21 provides modulated signal radiation 24 at wavelength λ.The radiation is coupled into transmission fiber 231, propagates todispersion compensator 20, undergoes dispersion compensation, followedby transmission through optional transmission fiber 232 to receiver 22.Conventional components such as amplifiers, filters, isolators, routersetc. are not shown.

It will be understood that, at least in principle, dispersioncompensation can take place at any point in an optical fibercommunication system. However, in many cases there will be dispersioncompensation just before detection of the signal.

Fiber according to the invention not only can be used for dispersioncompensation but can also be designed to provide dispersion slopecompensation over a range of wavelengths, exemplarily a range greaterthan 10 nm and including λ_(o).

As mentioned above, DC fiber according to the invention has thedesirable property of relative immunity with respect to (typicallyunavoidable) minor parameter variations.

FIG. 4 pertains to DC fiber substantially as shown in FIG. 1, with curve41 showing the nominal dispersion D_(nom) (λ) of fiber with the nominalrefractive index profile (i.e., parameters as shown in FIG. 1). Curves42 and 43 show the dispersion of fiber with Δ respectively increased by2% and decreased by 2% from Δ_(nom). Curves 44 and 45 show thedispersion of fiber with outside diameter (and therefore all radicaldimensions) respectively increased 1% and decreased 1% from the nominalvalue. As can be seen from FIG. 4, the departures of the dispersion fromD_(nom) (λ) over the range λ_(max) ±50 nm are relatively small, lessthan 50%.

The results of FIG. 4 should be compared with those of FIG. 5, whichshows dispersion data for an exemplary prior art DC fiber, with stepindex core (7.1 μm diameter, 1.8% Δ), without index ring. Curve 51 showsthe dispersion of the nominal fiber (i.e., parameters as stated above),curves 52 and 53 show the dispersion of fiber with Δ respectivelyincreased by 2% and decreased by 2% from the nominal design, and curves54 and 55 show the dispersion of fiber with outside diameterrespectively increased 1% and decreased 1% from the nominal design.Clearly, the exemplary prior art fiber is much more sensitive toparameter variations than is the exemplary DC fiber according to theinvention.

FIG. 6 shows the total delay for 100 km of commercially available singlemode transmission fiber (5D® fiber) together with an optimal length ofDC fiber substantially as shown in FIG. 1. By "optimal length" we meanthe length of DC fiber that yields the minimum delay at the operatingwavelength. Curve 61 shows the delay spectrum of the combination of 5Dfiber and the nominal design DC fiber of FIG. 1, curves 62 and 63 showthe delay spectrum of the 5D fiber together with, respectively, DC fiberwith 2% increased Δ and 2% decreased Δ, and curves 64 and 65 show thedelay spectrum of the 5D fiber together with, respectively, DC fiberwith 1% increased and decreased fiber outside diameter. All parametervariations are with respect to the exemplary nominal design. The resultsof FIG. 6 show that DC fiber according to the invention cansubstantially eliminate (e.g., about 3% residual delay) the delay ofconventional transmission fiber over a significant spectral region(e.g., 30 nm) in the presence of significant parameter variations.

FIG. 7 shows corresponding data for the above discussed exemplary priorart DC fiber (7.1 μm core diameter, 1.8% Δ, step index core, no indexring). Curve 71 pertains to the nominal design, curves 72 and 73 pertainto Δ increased by 2% and decreased by 2%, respectively, and curves 74and 75 pertain to outside diameter increased and decreased,respectively, by 1%. As can be seen, for decreased Δ and/or diameter,the delay is significant, and dispersion compensation is relativelyineffective.

A further advantage of preferred DC fiber according to the invention isrelatively low bending loss, compared with at least some prior art DCfibers. FIG. 8 shows the LP₀₂ mode effective index as a function ofwavelength, for the DC fiber of FIG. 1 (curve 81), and for thepreviously discussed prior art DC fiber (curve 82). Curve 83 is therefractive index of vitreous silica, the conventional outer claddingmaterial. As can readily be seen, the difference between curves 82 and83 is relatively small at any wavelength in the range 1.5 μm to 1.6 μm,indicative of susceptibility of the prior art fiber to bending loss. Ascan also be seen, the difference between curves 81 and 83 is relativelylarge (e.g., greater than 0.002) over the same spectral region,indicative of relatively low bending loss of the DC fiber according tothe invention.

More generally, in preferred DC fibers according to the invention thenominal refractive index profile is selected such that, over thewavelength range λ_(max) ±50 nm, (n_(eff) -n₁,nom)/n_(eff) is greaterthan 0.1 Δ_(nom) (λ), where n_(eff) is the mode effective index of theLP₀₂ mode.

Those skilled in the art will know that, for a given fiber design andwavelength, the effective index of a given mode can be readilydetermined, as can other fiber properties such as dispersion. See, forinstance, T. Lenahan, Bell System Technical Journal, Vol. 62, p. 2663(1983), incorporated herein by reference.

The invention claimed is:
 1. An article comprising a length ofdispersion-compensating optical fiber that supports radiation of apredetermined wavelength λ_(o) in a fundamental mode LP₀₁ and a higherorder mode, wherein the dispersion-compensating fiber has a nominalrefractive index profile includinga) a core of diameter d_(c),nom andmaximum refractive index n_(c),nom ; and b) an inner cladding regionthat contactingly surrounds the core and has refractive index n₁,nomless than n_(c),nom, with (n_(c),nom -n₁,nom)/n_(c),nom designatedΔ_(nom) ; where c) at least a portion of the length ofdispersion-compensating fiber has a refractive index profile thatdiffers from the nominal refractive index profile and includes a core ofdiameter d_(c) and maximum refractive index n_(c), and further includesan inner cladding region that contactingly surrounds the core and hasrefractive index n₁, with (n_(c) -n₁)/n_(c) designated Δ, where at leastone of d_(c), n_(c) and Δ differ from d_(c),nom, n_(c),nom and Δ_(nom),respectively; and where d) the dispersion-compensating fiber having saidnominal refractive index profile has nominal dispersion D_(nom) (λ), andsaid portion of the length of dispersion-compensating fiber hasdispersion D(λ)≠D_(nom) (λ), where λ is the wavelength; CHARACTERIZED INTHAT the nominal refractive index profile is selected such that e) thelength of dispersion-compensating fiber supports LP₀₁ and at least ahigher order mode LP₀₂ at λ_(o), and D_(nom) (λ) is more negative than-200 ps/nm.km over at least a wavelength range λ_(max) ±50 nm, whereλ_(max) is the wavelength at which |D_(nom) (λ)| is maximum; f)|D(λ)-D_(nom) (λ)| is less than |0.5 D_(nom) (λ)| at every wavelength λin the range λ_(max) ±50 nmi) for d_(c) that differs from d_(c),nom by1% or less, or ii) for Δ that differs from Δ_(nom) by 2% or less, oriii) for d_(c) that differs from d_(c),nom by 1% or less and Δ thatdiffers from Δ_(nom) by 2% or less, where the vertical bars before andafter a quantity indicate the absolute value of the quantity between thevertical bars; g) λ_(o) is a wavelength in the range λ_(max) ±50 nm; andh) the nominal refractive index profile is selected such that D_(nom)(λ) is substantially all in LP₀₂.
 2. Article according to claim 1,wherein λ_(o) ≦λ_(max).
 3. Article according to claim 1, wherein thenominal refractive index profile is selected such that (n_(c),nom·d_(c),nom ·√Δ_(nom) )/λ_(o) is greater than 0.55.
 4. Article accordingto claim 1, wherein the nominal refractive index profile is selectedsuch that, over the wavelength range λ_(max) ±50 nm, (n_(eff)-n₁,nom)/n_(eff) is greater than 0.1 Δ_(nom) (λ), where n_(eff) is aneffective refractive index of the LP₀₂ mode.
 5. Article according toclaim 1, wherein the dispersion-compensating fiber is silica-baseddispersion-compensating fiber comprising a Ge-doped core, an F-dopedinner cladding that contactingly surrounds the core, and an undoped orGe-doped refractive index ring that contactingly surrounds the innercladding.
 6. Article according to claim 1, wherein the article is anoptical fiber communication system that comprises a transmitter forproviding signal radiation of wavelength λ_(o), a receiver for receivingthe signal radiation, and an optical fiber transmission path for signalradiation-transmissively connecting the receiver and transmitter,wherein the optical fiber transmission path comprises said length ofdispersion-compensating fiber.
 7. Article according to claim 6, whereinλ_(o) is approximately 1.55 μm.
 8. Article according to claim 7, whereinthe nominal refractive index profile of the dispersion-compensatingfiber is furthermore selected such that the dispersion-compensatingfiber provides dispersion slope compensation over a range of wavelengthsgreater than 10 nm, wherein λ_(o) is in said range of wavelengths.